A membrane-bound antigen of chick red cells, characteristically present before hatching and disappearing with post-hatch development, has been shown to be inducible with avian myeloblastosis virus (AMV) leukemogenesis. Using a combination of immunological and scanning electron microscopic methods, the number of molecules of chicken fetal antigen (CFA) on the surface of newly-hatched chick red blood cells and adult chicken red blood cells has been ascertained. We propose to develop a radioimmune assay (RIA) for the antigen for diagnostic and investigative purposes. Further studies are designed to investigate the following: a. The extent to which leukemic cells undergo generalized genetic derepression. This will be investigated by determining levels of enzymes or structural proteins known to vary during development. b. The species distribution of CFA. c. Cellular distribution and specificity of CFA in the chicken. The distribution and sequence of disappearance of the antigen with time will be determined for specific cells and tissues. d. The relationship between CFA and AMV. We have shown that CFA is an unlikely receptor site for AMV. However, there may be sequence similarity between AMV and the genes coding for CFA. e. Mitochondrial DNA of chicken leukemia. We shall determine whether or not chicken leukemia show the mitochondrial DNA dimers present characteristically in the human leukemias. f. The cross-reactivity between antisera directed against CFA and human leukemias. Pilot studies have shown that the antiserum used to detect CFA also detects more than 50% of human leukemias. The possibility of the use of antiserum as a sensitive reagent for the detection of human leukemia will be further explored.